Ku is a complex of 7OkDa and 86kDa proteins, found in the nucleus of human cells where it binds DNA and is the target of autoantibodies in several autoimmune conditions. We have now demonstrated that both the 7OkDa and 86kDa components of Ku are present in small amounts on the cell surface and that they are integral membrane proteins. By use of antibodies to synthetic peptides, derived from the 7OkDa protein sequence and fluorescein-activated cell sorting we demonstrated that several different domains are exposed on the cell surface. Synthetic peptides were used to identify the domains of the 7OkDa protein which bind DNA. By slot blot analysis one peptide of 15 amino acids (peptide 25A) bound DNA with very high affinity and was able to inhibit the binding of the recombinant 7OkDa protein to DNA. Another peptide (peptide 4) although contains a number of basic amino acids and is contiguous with one of the leucine zipper regions in the protein bound DNA weakly. The use of short peptides to identify DNA binding domains is original and may have broad applicability to other DNA binding proteins. Efforts are underway to understand the normal cellular function of this protein. Studies are continuing on cloning of autoantigens involved in diabetes mellitus. Two cDNA clones, (which are preferentially expressed in human insulinoma), were isolated from an insulinoma glucagonoma subtraction library. One clone, IG-3, detected a transcript of 2.4 kb in human insulinoma tissue. Another cDNA clone IG-20 detected a 5.0 kb transcript in human insulinoma, and at a lower level in the brain. Partial DNA sequence analysis of this clone revealed no homology to known gene sequences and thus represents a unique gene. Studies are underway to characterize these genes.